1. Field of the Invention
The present invention relates to a liquid crystal driver used for correcting tilt of an optical head used in an optical disk drive.
2. Description of Prior Art
Recently, it is proposed to correct tilt in an optical head in an optical disk drive with a liquid crystal element in order to correct aberration (Japanese Patent laid open Publications 10-79135/1998 and 11-3531/1999). The tilt is an inclination of the optical axis of an optical head relative to an optical disk. Electrodes in the liquid crystal element are provided as a plurality of divided areas of predetermined shapes, and voltages applied to the areas are controlled for tilt correction by changing the phase difference of a transmitting laser beam. Spherical aberration of object lens is a problem for higher density recording of optical disk when a short wavelength laser is used for a lens of high numerical aperture (NA). Spherical aberration is also corrected with the liquid crystal element.
When tilt correction is performed only in one direction, the liquid crystal element has divided areas of generally long areas and the voltages applied to the areas are controlled. When tilt correction is performed in two directions, the liquid crystal element has divided areas of generally fan-like shapes extending in two directions, and a central area around the optical axis is provided as a common area. In this case, the voltages applied to the areas are controlled similarly to the correction in one direction.
In order to apply voltages, an operational amplifier or a pulse width modulation circuit is used generally. When tilt correction is performed in one direction and in two directions, as mentioned above, a driver circuit is provided for each of the areas in the liquid crystal element.
As to the driver circuit, it is necessary to provide a driver circuit for each of the divided areas in the liquid crystal element. Then, the circuit scale of the driver circuit becomes large. Further, a large number of connections for driving the liquid crystal element are needed in an interface between the optical head with the liquid crystal element and the circuit board.
Further, when tilt correction is performed in two or more directions, it is desirable to correct the tilt for each direction independently. However, it is troublesome to determine voltages applied to the areas except the common area for each direction by taking into account the voltage applied to the common area. Recently a digital signal processor is used to determine the applied voltages, and the number of the steps becomes large in the software processing in the digital signal processor.
In a prior art optical head, a liquid crystal element is mounted not on an actuator which is a moving component, but on a fixed component. This is adopted in order to improve sensitivity on focusing and tracking by making the optical head compact and by using a light-weight actuator. Tilt caused by inclination of the object lens in radial and tangential directions are corrected. However, when the object lens is moved in the radial direction, it is a problem that the optical axis of the object lens is shifted relative to the liquid crystal pattern, so that performance of aberration correction is deteriorated.
An object of the present invention is to provide a liquid crystal driver which performs tilt correction in a plurality of directions independently of each other.
Another object of the present invention is to provide a liquid crystal driver which corrects spherical aberration of the object lens.
A further object of the present invention is to provide an optical head having a fixed liquid crystal element which performs tilt correction in the radial and tangential directions and improves aberration correction performance when the object lens is moved in the radial direction.
In one aspect of the invention, in a liquid crystal driver, a periodic waveform generator generates a periodic waveform signal, a phase shifter receives the periodic waveform signal from the periodic waveform generator and shifts phase thereof according to an instructed value, and an inverting element inverts an output signal of the phase shifter. A potential divider comprises a plurality of resistors connected in series, and the output signal of the phase shifter and the output signal of the inverting element are connected to two ends of the potential divider. A liquid crystal element comprises electrodes of a plurality of areas and a common electrode opposing the electrodes, wherein the common electrode is connected to the periodic waveform signal outputted by the periodic wave generator and the plurality of areas are connected to output voltages of the potential divider. Thus, tilt correction is performed with a simple structure of liquid crystal driver. When tilt is corrected in a plurality of directions, the liquid crystal driver may have a plurality of sets of the phase shifter, the inverting element and the potential divider for each direction.
In another aspect of the invention, a liquid crystal driver has a plurality of phase shifters, inverting elements, and potential drivers. A liquid crystal element comprises electrodes of a plurality of areas extending in the plurality of directions and a common electrode opposing the electrodes. The common electrode is connected to the periodic waveform signal outputted by the periodic wave generator, and the plurality of areas are connected to output voltages of the plurality of potential dividers.
In a further aspect of the invention, an optical head comprises a light source, an object lens for converging a light beam emitted by the light source, and a liquid crystal element arranged between the light source and the object lens. The liquid crystal element comprises electrodes of a plurality of areas in a plane perpendicular to an optical axis of a light beam reflected from an optical disk and a common electrode opposing the electrodes via a liquid crystal layer. The electrodes comprise a first electrode group used for correcting the light beam transmitting when no shift of the object lens occurs relative to the optical axis, an at least one second electrode provided adjacent to the first electrode groups in a first direction along which the object lens is shifted, and at least one third electrode provided adjacent to the first electrode groups in a second direction opposite to the first direction.
An advantage of the present invention is that a liquid crystal driver has a simple structure of a driver circuit for each direction.
Another advantage of the present invention is that tilt correction in a plurality of directions can be performed independently of each other without interference between each other.
A further advantage of the present invention is that the optical head performs tilt correction more precisely with high reliability.